1. Field of the Invention
This invention relates to a DC operated alkali metal vapor arc discharge lamp. More particularly, this invention relates to a DC operated sodium vapor arc discharge lamp comprising an elongated cylindrical arc tube having an electrode sealed in each end to form a cathode end and an anode end and containing a sodium amalgam and a noble starting gas, with the temperature at the cathode end at least 50.degree. C. greater than the temperature at the anode end and a cathode to anode sodium pressure ratio no greater than 5 during operation.
2. Background of the Disclosure
High intensity alkali metal vapor arc discharge lamps, such as high pressure sodium (HPS) arc discharge lamps are widely used for outdoor lighting because of their high efficacy as measured in lumens per watt. However, lighting systems employing HPS lamps often exhibit noticeable buzzing, light flicker and stroboscopic effects during operation from AC sources which can be annoying to the observer. This is especially noticeable with a standard 50-60 Hz line source. There are some specialized applications where only DC power is available such as in mines or on heavy equipment. DC operation of such lamps, particularly on steady DC, will avoid the flickering problem but create other problems associated with cataphoresis. Those skilled in the art know that cataphoresis is a problem with any continuous DC operated lamp having multi-component vapors and in the case of an arc discharge lamp containing alkali metal vapor and mercury or noble gas, exhibits itself as a higher concentration of the alkali metal at the cathode or negative end of the arc tube or chamber due to the unidirectional force toward the cathode that the electric field exerts on the alkali metal ions. Thus, the alkali metal partial pressure at the cathode end of the arc tube is greater than that at the anode end. This results in a difference in both the color and intensity of the emitted light along the length of the arc tube and is more noticeable with longer and narrower arc tubes. This does not normally occur in AC operation wherein the alkali metal pressure is fairly uniform or constant along the length of the arc tube.
The cataphoresis phenomenon also occurs with low pressure metal vapor arc discharge lamps, such as fluorescent lamps which use mercury vapor and a noble gas in the light-emitting arc discharge. Some attempts have been made to overcome cataphoresis in DC operated fluorescent lamps. U.S. Pat. No. 3,117,248 discloses a feedback tube between the anode and cathode ends of the lamps and also suggests counteracting cataphoresis by increasing the wall temperature or current density. In U.S. Pat. No. 3,617,792 a highly loaded and unsealed glass tube inside a fluorescent lamp envelope is employed to counteract cataphoresis. U.S. Pat. No. 4,698,549 discloses the use of an indium amalgam behind the anode in order to maintain a more even mercury distribution in a DC operated fluorescent lamp, but this will not work with an alkali metal vapor arc discharge lamp such as an HPS lamp. Moreover, it is not practical to use a feedback tube between anode and cathode ends for an HPS lamp nor is it practical to use an unsealed tube inside the arc tube to counteract for cataphoresis in such lamps. Hence, there is a need for a DC operated, high intensity alkali metal vapor arc discharge lamp and particularly one having an efficacy proximate that of an AC operated lamp of the same wattage.